Vinyl chloride homo- and copolymers stability

ABSTRACT

Homopolymers and copolymers of vinyl chloride with vinylidene chloride, vinyl acetate, propylene, acrylates and methacrylates are treated as a slurry with A. A 0.1-10 PERCENT SOLUTION OF AN ALKALINE, WATER-SOLUBLE HYDROXIDE IN ORDER TO REMOVE UNDESIRABLE SIDE PRODUCTS OF POLYMERIZATION TENDING TO DEGRADE THE POLYMER, AND B. A PHENOLIC ANTIOXIDANT SUCH AS A BIS OR TRIS PHENOL OR A SALT, THIO- OR CARBOXYLIC ACID DERIVATIVE THEREOF.

United States Patent [72] inventor Laurence F. King Mooretown. Lambton,Ontario, Canada [2| Appl. No. 804,706 [22] Filed Mar. 5, i969 [45]Patented Nov. 16,19" {73] Assignee Esso Research and Engineering Company[54] VINYL CHLORIDE HOMO- AND COPOLYMERS STABILITY 3 Claims, No Drawings[52] US. Cl 260/45.85, 260/45.7 R, 260/459 R, 260/4595, 260/928 A [51]Int. Cl C08f 45/58 [50] Field of Search 260/45.7 R,

References Cited Primary Examiner-Donald E. Czaja Assistant ExaminerV.P. Hoke Attorneys-Chasan and Sinnock and Michael N. Meller ABSTRACT:Homopolymers and copolymers of vinyl chloride with vinylidene chloride,vinyl acetate, propylene, acrylates and methacrylates are treated as aslurrywith a. a 0.l-l0 percent solution of an alkaline, water-solublehydroxide in order to remove undesirable side products of polymerizationtending to degrade the polymer, and

b. a phenolic antioxidant such as a bis or tris phenol or a salt, thioorcarboxylic acid derivative thereof.

BACKGROUND OF THE INVENTION 1. Field of Invention This invention relatesto the stabilization of resinous compositions containing primarily homoand copolymers of vinyl chloride, hereinafier designated as PVCcompounds. More specifically the invention relates to the processabilityof PVC as indicated by the dynamic mill stability of compositions madeby suspension and emulsion polymerization reactions.

PVC, particularly in rigid formulations, is more difficult to processthan many other resins because of the tendency for HCI to be liberatedat the high temperatures required. For each mole of HCl lost an olefinicbond appears in the polymer backbone, and after about seven conjugateddouble bonds have been formed, probably by a zipper" mechanism, colorformation due to the polyene structure becomes appreciable. Conventionalstabilizers-organotins, heavy metal salts, epoxides, phosphites,etc.-retard thermal degradation during processing, but do not eliminateit entirely. A recently published comprehensive review on the mechanismof PVC degradation by W. C. Geddes in Rubber Chemistry and Technology40, No. l, p. 177 (1967) points out that the susceptibility to thennalbreakdown is due primarily to structural irregularities in the polymer.These structural irregularities affecting the thermal stability of PVCcan be listed as follows:

a. initiator residues b. unsaturated end groups c. branch points d.random unsaturation e. oxidized structures f. head-to-head units g.stereoconfiguration h. extraneous impurities The atmosphere in contactwith the resin during any processing sequence also has a marked effecton the rate surface area of the polymer being processed variesconsiderably from one machine to another. On a mill or calendar, theratio is high whereas in a press it is relatively low. Because ofoperating differences, laboratory test procedures, particularly statictests such as oven stability, do not always correlate well withcommercial practice.

2. Description of the Prior Art ln an effort to cure these problems, theprior art has attempted to stabilize PVC degradation such as for exampleby the use of multivalent metal phenolates or carboxylates withbisphenols as described in Darsa, U.S. Pat. No. 3,225,001, or by theaddition of alkali metal phosphates such as potassium or lithiumphosphates as set out in Haefner et al., U.S. Pat. No. 2,868,765. Thesematerials, while highly useful for product stability in use of suchmaterials as wire and cable coatings and the like where high-temperaturecharacteristics are necessary, do not serve very well during processingof these compounds and thus to give dynamic mill stability it isnecessary to use other and more effective antioxidant formulations.Furthermore, inorganic salts such as the phosphates, since they arecompletely insoluble in PVC, impart undesirable haziness to clear,transparent compounds and may be leached from the resin by water duringend use.

The inventors investigation has shown that the function of antioxidantsherein is twofold: (a) during treatment of a resin with specifiedchemical agents such as alkali metal hydroxides, it prevents colordegradation which would render the resin useless for subsequentcompounding; and (b) during the processing step. Phenolic-typeantioxidants are more effective in some formulations than in others. Inflexible formulations containing metallic soaps (which serve as bothlubricant and stabilizer) antioxidants are extremely effective inpreventing the compound from sticking to metal surfaces duringprocessing. In most rigid compounds, in which free fatty acids arealmost universally used as the major lubricant, antioxidants are withouteffect except when employed in conjunction with an alkaline reagent, aswill be shown herein.

Suitable chemical treatment of PVC under relatively mild conditionsinduces some controlled dehydrochlorination of the resin and removestrace impurities. The result is improved clarity of the product in someinstances and better thermal stability as indicated by most tests.Aqueous alkaline solutions in the presence of an antioxidant appear torefine" the resin by removing the most labile chlorine atoms along withsuspending and emulsifying agent residues and other extraneousimpurities such as initiator residues without adversely affectingparticle size, porosity or color.

It is well known that the metallic stabilizer and the lubricant used areboth important in determining processability of a compound. In somecases the presence of an antioxidant was found to have a further, majoreffect on heat stability; in others, changes induced by alkali treatingwere greater, while in still other formulations, both methods oftreatment appeared advantageous.

This invention is based on the theory which applicant hereby isadvancing merely as a means of explanation, but does not wish to bebound by, that the labile chlorine atoms are removed from the PVC by acontrolled dehydrochlorination process and immediately replaced withother more stable atoms. Thus, the thermal stability of PVC is greatlyenhanced without concomitant color degradation. This is in part thetheoretical basis of stabilization by metal carboxylates, mercaptides,etc., (which are normally added at a later stage) whereby ester ormercapto groups become chemically bonded to the polymer chain duringprocessing.

In practice, controlled dehydrochlorination of PVC is extremelydifl'icult. Once started, dehydrochlorination becomes very rapid atelevated temperatures and color formation due to the polyene structuresoon becomes appreciable unless ex ceptional treating methods areemployed.

It is the purpose, therefore, of this invention to set forth a novel wayof treating PVC compounds subsequent to polymerization with acombination of an alkaline hydroxide in aqueous solution and anantioxidant so as to dehydrochlorinate PVC without color degradation andfurther, to impart thereby improved processability characteristics tothe resin after compounding in the usual manner, as determined bydynamic mill stability.

SUMMARY OF THE lNVENTlON Briefly this invention relates to a process ofimproving the heat stability of PVC-type compounds during processing bytreating them with 0.1 to 10 percent of a water soluble hydroxideselected from the group consisting of sodium hydroxide, potassiumhydroxide, ammonium hydroxide, lithium hydroxide, calcium hydroxide andbarium hydroxide in a solution having a pH in the range of 7 to 12 inorder to induce controlled dehydrochlorination and to remove undesirableside products of polymerization tending to degrade PVC polymers. Theseundesirables would include polyvinylalcohols, gelatin, methyl cellulose,carboxy methyl cellulose, hydroxy ethyl cellulose which are commonlyused as suspending agents and remain adsorbed to or grafted on thepolymer, and other hydrolyzable materials which are produced as part ofthe polymerization process, such as initiator residues (acetyl or estergroups) attached to the polymer, or in the case of copolymers, unreactedcomonomer such as vinyl acetate. The secondary treatment involves theadditional stabilizing action of a phenolic antioxidant such as a bis ortris phenol or a derivative thereof.

It has been found by the inventor, quite surprisingly, that heattreating a commercial suspension of PVC resin in the form of a slurry atrelatively low temperatures ranging from fl0-150 C. with a diluteaqueous solution of an alkali such as sodium or ammonium hydroxide in aninert in an inert atmosphere results in a greatly accelerated rate ofdehydrochlorination as compared to using weaker bases.

Depending on the temperature and the amount and kind of alkali used, thedehydrochlorination rate is to 25 times that obtained during the sameheat treatment under nitrogen by prior techniques. In spite of the lossof HCl resin color and lithium, barium and calcium hydroxide would alsobe useful.

As the phenolic-type antioxidant there may be employed compounds such asl. 4,4 isopropylidene bisphenol pH of aq. mediu Temperature, 0. Contacttime, hrs. Dehydrochlorinati Resin color Formulation;

Resin Alkyl tin mereaptide 4 Stearic acid 12-hydroxy stearic acidEvaluation:

Oven test life, 177 0., mins Dynamic mill stability, 177 C Clarity loss,mins. Life, mins other particle properties are not adversely affected.In fact, 5 IL 4 bis (p hydroxy phenyn pentanoic acid resin particleporosity is often increased due to the penetrating "L thiobis (646".buty] m creso|) action of the aqueous solution and this is desirable forbetter tetrakis [methylene dHerL bmyl' 4' hydroxy processability n habsgrption of P fifii i i'. phenyl) propionate] methane In the same treaing tep proposed. the p rti lly The process parameters for thepostpolymerization antioxdehydrochlorinated resin is reacted with suchantioxidants as id i treatment o ld b a e e atur range fr m mbi ntphenols to introduce chemical groups on the PVC backbone t 150 C, withef red te erature range betwe n 35 r absorb them strongly on the polymerthereby providing inand 75 C. The pressures at which the reaction wouldbe perternal stabilization. Although the exact mechanism iS unkformedwould range from atmospheric to 200 p s i. and nown, color degradationis effectively prevented, particularly preferably from atmospheric to100 psi. The contact times at a pH value between 7 and 10. Other(desirable) chemical may range from 0.1 to hours and preferably from 0.1to 2 reactions such as decomposition of suspending agent residues hours.are not inhibited, as shown by the fact that significant amounts Thecritical p range is between 7 and I2 for of acetaldehyde and acroleinare evolved from polyvinyl al- 20 dehydrochlonnation of PVC. Anythingover a pH of 12 would cohol suspending agents and methanol from methylcellulose. u Removal of these contaminants materiall im roves thethertend to degrade the polymer by Stamng off the upper y p ma]Stability ofthe resin. mechanism with too much polyene formationbecoming A evldent. The preferred pH range 15 8 to 9 at 35-75 C. and aThe ro e i e l contemplates treating under contact time of about 2hours. A pH range of between 9 and nitrogen a water-suspended PVC resinwith 0.1 to 10 percent 12 can be p y if the temperature is below and/orof resin of alkaline reagent and an amount of approximately the Comatime is 1655 than about 2 h Conversely at high 005 to 0.5 parts of a bisor tris phenol antioxidant at a temtemperatures the pH range should bebetween 8 and 9 and the perature ranging from 40 to 150 C. and for aperiod of time contact time only about 0.1 hour. Resin color degradationis ranging from 0.1 to 20 hours, followed by water washing to the ts ysreumove texcesls Ialkalinei reagent, alkali metal chlorides and Thisprocess for both the rigid and flexible PVC 0 er er so u Pro uctspounds, provides better clarity and heat stability during The resinemployed may be any homopolymer of vinyl processingas well as betterresistance to oxidation. chloride or copolymers of vinyl chloride withvinylidene Examples f f l materials made f pv compounds chloride Vinyl Py e alkyl vinyl ether acrylate processed according to this inventioninvolve plastic bottles, and mfithacrylate d Qther type m ll m pipe,articles, calendered film, floor tile, blown film, etc. The slurrymedium would generally be water and it would be employed in 0.5 to 2parts per part of resin depending upon the viscosity of the medium.DESCRIPTION OF THE PREFERRED EMBODIMENT The alkali employed wouldgenerally be sodium hydroxide, This invention and its advantages will bebetter understood but other similar hydroxides such as potassium,ammonium,- by reference to the following examples:

Examples 1 2 3 4 5 6 7 8 9 10 11 12 Treat:

PVC homopolymer PVC copolymer Water 1 Polyvinyl chloride, 15% poluvinylacetate. 2 5 parts of 29% NH OH solution a Tetrakis [methylene 3-(3',5di-tert. butyl, 4 hydroxy phenyl) propionate] methane.

Sn where R and R are alk l l'Oll s. R/ \SR y l, I

5 Time to reach 0.1 absorbance (79% light transmission) measured byspeetropothometer at 476 m on specimen 9.010" thick.

Example I is an untreated homopolymer compounded with a stabilizer and alubricant in the usual manner. The data shown were obtained in the ovenstability and dynamic mill stability tests. Example 2 is the same resintreated for 2 hours with water alone at 80 C. and shows pinkdiscoloration due to thermal degradation which would render itunsuitable for processing and fabrication. Example 3 shows that sodiumhydroxide treatment without antioxidant gives similar color degradation,and example 4 indicates the beneficial results of sodium hydroxidetreatment in the presence of an antioxidant whereby discoloration isprevented and the heat stability is significantly improved, as measuredby both static and dynamic tests. Examples 5, 6 and 7 demonstrate aparallel efi'ect with ammonium hydroxide solution and further indicatethe extent of dehydrochlorination achieved. The increase in dynamic milltest life for 5 and 7 as compared to example i is of the order of 40percent. in examples 8 and 9 a slight modification in the compoundingingredients was made by replacing stearic acid with l2-hydroxy stearicacid and the improvement due to ammonium hydroxide treating in thepresence of the phenolic antioxidant is about the same (40 percent). Theremaining three examples 10, l l, and 12 are for a vinyl chloride-vinylacetate copolymer. They show the effect of sodium hydroxide treating attwo pH levels, 12 and 10, and indicate an improved performance at thelower pH level. The time for loss of clarity was extended by 50 to 100percent which is of the greatest importance in processing of all PVCresins.

It is to be understood that this invention is not limited to specificexamples set forth herein, which have been offered merely asillustration, and that modifications maybe made without departure fromthe spirit and scope of the appended claims.

What is claimed is:

l. In a process for producing a stabilized polymer selected from thegroup consisting of homopolymers of vinyl chloride and copolymers ofvinyl chloride with vinylidene chloride, vinyl acetate, propylene,acrylates and methacrylates prepared by suspension polymerization andusing suspending agents selected from the group consisting of polyvinylalcohols, gelatin, methyl cellulose, carboxy methyl cellulose andhydroxy ethyl cellulose, the improvement which comprises hydrolyzing theresidues of said suspending agents and preventing color degradation bytreating an aqueous suspension of said polymer in an inert atmospherewith:

a. a 0.1 to 10 percent aqueous solution of an alkaline hydroxide havinga pH in the range of 7.0 to 12.0, in combination with,

b. 0.05 to 5 parts of a phenolic antioxidant selected from the groupconsisting of his and tris phenols and their salts,

at a temperature of from 40 to C. for 0.1 to 20 hours followed by waterwashing and recovery of the polymer.

2. The process of claim 1 wherein the alkaline hydroxide is selectedfrom the group consisting of sodium hydroxide, potassium hydroxide,ammonium hydroxide, lithium hydroxide, calcium hydroxide and bariumhydroxide.

3. The process of claim 1 wherein said phenolic antioxidant is tetrakis[methylene 3-(3,5' di-tert. butyl, 4' hydroxy phenyl)propionate] methaneand said alkaline hydroxide is sodium hydroxide or ammonium hydroxide.

l t i t

2. The process of claim 1 wherein the alkaline hydroxide is selectedfrom the group consisting of sodium hydroxide, potassium hydroxide,ammonium hydroxide, lithium hydroxide, calcium hydroxide and bariumhydroxide.
 3. The process of claim 1 wherein said phenolic antioxidantis tetrakis (methylene 3-(3'',5'' di-tert. butyl, 4'' hydroxyphenyl)propionate) methane and said alkaline hydroxide is sodiumhydroxide or ammonium hydroxide.